1. Field of the Invention
The present invention relates to a control system for an automobile automatic transmission and, more particularly, to an automatic transmission control system in which shelf pressure is controlled by a pressure controller, such as an accumulator, differently between locking and unlocking of a frictional coupling element of the automatic transmission.
2. Description of Related Art
Typically, automatic transmissions for automobiles have a torque converter and a transmission mechanism which includes a plurality of frictional coupling elements selectively coupled and uncoupled to place the automatic transmission into any one of a plurality of desired transmission modes and gears according to driving conditions. Selectively coupling and uncoupling of these frictional coupling elements is performed by the use of hydraulically operated actuators. In such an automatic transmission, which has, for instance, four forward gears, i.e. first to fourth gears, a specific one of these frictional coupling elements is coupled or locked for providing second (2nd) and fourth (4th) gears, which is hereafter referred to as a 2/4 frictional coupling element, and another specific frictional coupling element is released or unlocked for third (3rd) and 4th gears, which is hereafter referred to as a 3/4 frictional coupling element.
If the 2/4 frictional coupling element is configured by a band brake (a 2/4 band brake), a rotative counter element, which is braked or locked by the 2/4 band brake can possibly rotate in opposite directions between a 1-2 gear shift-up from the 1st gear to the 2nd gear and a 3-4 shift-up from the 3rd gear to the 4th gear. In such a configuration of the automatic transmission, if the rotative counter element rotates in a so-called leading direction, in which it is effectively braked by the 2/4 band brake, during the 1-2 shift-up from the 1st gear to the 2nd gear, it rotates in a trailing direction which is the trailing direction of the band brake and in which the 2/4 band brake is less effective, during the 3-4 gear shift. In such a case, because the 2/4 band brake does not provide its own servomechanism function during the 3-4 gear shift, an actuator needs a higher level of locking pressure to activate the 2/4 band brake for causing the 3-4 gear shift-up as compared with causing the 1-2 gear shift-up.
On the other hand, in an attempt to reduce shocks generated in an automatic transmission of this type when the frictional coupling elements are locked, it is typical that a hydraulic control circuit is designed and adapted to activate the actuator with hydraulic pressure changing in level by way of what is called "shelf pressure". The term "shelf pressure" as used in this specification shall mean and refer to a transitional level of pressure decreasingly changing at an incline or gradient more gentle before and after the change. This shelf pressure is generally developed by means of an accumulator in a pressure line leading to the actuator.
However, there occur some problems including responsiveness and shift shocks of the automatic transmission controlled by such a hydraulic control circuit equipped with the accumulator for producing shelf pressure in connection with the 2/4 frictional coupling element which is locked for more than one transmission gear. Specifically, in the case wherein the 2/4 frictional coupling element needs to be activated with a locking pressure at a level higher for the 3-4 gear shift than for the 1-2 gear shift, if the accumulator produces the shelf pressure suitable in level for the 1-2 gear shift then there is caused a deterioration of responsiveness of the automatic transmission to a demand of the 3-4 gear shift due to insufficiency of the locking pressure. Conversely, if the accumulator produces the shelf pressure suitable in level for the 3-4 gear shift, the 2/4 frictional coupling element is locked abruptly in excess during the 1-2 gear shift, so as to cause undesirable shift shocks.
In order to avoid such a problem for the automatic transmission shiftable to four forward gears, the hydraulic control circuit is provided with an accumulator having a locking pressure line leading to the 2/4 frictional coupling element and a back pressure chamber connected to a pressure line branching off from a locking pressure line leading to the 3/4 frictional coupling element. With this hydraulic control circuit, during a 3-4 gear shift from the 3rd gear with the 3/4 frictional coupling element having been locked to the 4th transmission gear, locking pressure having been introduced into the 3/4 frictional coupling element is supplied as back pressure into the back pressure chamber of the accumulator through the locking pressure line leading to the 2/4 frictional coupling element, so as to cause the accumulator to produce a high level of shelf pressure. On the other hand, during a 1-2 gear shift from the 1st gear with both 2/4 and 3/4 frictional coupling elements having been unlocked or released to the 2nd gear, since the 3/4 frictional coupling element is held unlocked or released, the locking pressure having been introduced into the 3/4 frictional coupling element is not supplied into the back pressure chamber of the accumulator, so as to cause the accumulator to produce a shelf pressure at a lower level as compared with during the 3-4 gear shift. In other words, a properly controlled shelf pressure is produced for both 3-4 gear shift, wherein the 2/4 frictional coupling element needs a high locking pressure, and 1-2 gear shift, wherein it does not needs a high locking pressure. Such an automatic transmission is known from, for instance, Japanese Unexamined Patent Publication No. 4 -136561.
The automatic transmission described in the above publication, which features an accumulator with a back pressure chamber provided in a locking pressure line leading to a specific frictional element is supplied with a locking pressure from another specific frictional, is still subject to a problem of shift shocks caused during gear shifts.
Describing the shift shock problem by way of example, even during a 4-3 gear shift from the 4th gear to the 3rd gear, the locking pressure supplied to the 3/4 frictional coupling element is introduced into the back pressure chamber of the accumulator connected to the locking pressure line leading to the 2/4 frictional coupling element as well as during a 3-4 gear shift. In the automatic transmission, when performing a 3-4 gear shift, the transmission gear mechanism operates in an opposite such that a rotative counter element is braked. Accordingly, in order for the transmission mechanism to perform a satisfactory 3-4 gear shift, the 2/4 frictional coupling element needs as a necessary torque a total torque of a torque assigned thereto and an inert torque of the rotative counter element. If the shelf pressure is at a rather high level for the purpose of providing the 2/4 frictional coupling element with a sufficient torque during the 3-4 gear shift, the 2/4 frictional coupling element is abruptly unlocked or released at the end of the period of the supply of shelf pressure during a gear shift in the reverse direction, namely the 4-3 gear shift. This can possibly cause excessive shift shocks. On the other hand, if the shelf pressure is established at a lower level so as to be suitable for the 4-3 gear shift, then the 2/4 frictional coupling element needs a long time to be locked during the 3-4 gear shift, resulting in a deteriorated feeling of gear shift and/or shift shocks due to an excessive drop in output torque.
These problems are caused not only in automatic transmissions having a band brake type of 2/4 frictional coupling element which is locked for a 3-4 gear shift and unlocked for a 4-3 gear shift, but also in automatic transmissions having an ordinary multi-plate type of frictional coupling element which is locked to provide one gear and unlocked to provide another gear if pressure varies by way of the same level of shelf pressure during the gear shift from the one gear to the other and during the gear shift from the other gear to the one gear. In addition, these problems are also caused if an element or clean, such as a control valve, other than accumulators is or are used to generate such shelf pressure.